1. Field of Invention
The present invention relates to a method of controlling the oxygen content in valve metal materials and, more particularly, to a method of controlling the oxygen content in powders of tantalum, niobium, and alloys thereof, useful in the production of capacitors, and in sintered anode bodies made from tantalum, niobium, and alloys thereof.
2. Description of the Related Art
Valve metals may be used to form wrought products, such as bars, plates, sheets, wires, tubes and rods, and preforms for subsequent thermo-mechanical processing. In addition, capacitors can be manufactured by compressing agglomerated tantalum powders to form a pellet, sintering the pellet in a furnace to form a porous body (electrode), which is sometimes followed by deoxidation of the electrode by reaction with a reactive metal, such as magnesium, and then subjecting the body to anodization in a suitable electrolyte to form a continuous dielectric oxide film on the sintered body.
As is known to those skilled in the art, valve metals generally include tantalum, niobium, and alloys thereof, and also may include metals of Groups IVB, VB, and VIB and alloys thereof. Valve metals are described, for example, by Diggle, in "Oxides and Oxide Films", Vol. 1, pages 94-95, 1972, Marcel Dekker, Inc., New York.
Tantalum and niobium are generally extracted from their ores in the form of powders. Tantalum powders, for example, that are suitable for use in high performance capacitors, can be produced by chemical reduction, such as sodium reduction, of potassium fluorotantalate. In this process, the potassium fluorotantalate is recovered from processed ore in the form of a dry crystalline powder. The potassium fluorotantalate is melted and reduced to tantalum metal powder by sodium reduction. The tantalum powder formed is then water washed and acid leached. Dried tantalum powder is then recovered, thermally agglomerated at temperatures up to about 1500.degree. C., and crushed to a granular consistency. Typically, the granular powder is then deoxidized in the presence of a getter material having a higher affinity for oxygen than the valve metal at elevated temperatures up to about 1000.degree. C., and is then acid leached to remove residual metal contaminants and their oxides. The powder is then dried, compressed to form a pellet, sintered to form a porous body, and subjected to anodization in a suitable electrolyte to form a continuous dielectric oxide film on the sintered body. In an alternative method, the powder is produced by hydriding a melted tantalum ingot, milling the hydrided chips, and dehydriding. In all cases it is possible, and sometimes desirable, to deoxidize the sintered anode pellet in a process similar to that described above for the powder.
Valve metal powders, particularly powders of tantalum, niobium, and their alloys, that are suitable for making capacitors should provide an adequate electrode surface area when compressed and sintered. The ufV/g of the capacitor is proportional to the surface area of the sintered porous body. The greater the specific surface area after the sintering operation, the greater the ufV/g. The purity of the powder is also an important consideration in its use in capacitor production. Metallic and non-metallic contamination can degrade the dielectric oxide film on the capacitors. While high sintering temperatures can be used to remove some volatile contaminants, the high temperatures may also shrink the porous body and its net specific surface area and, therefore, the capacitance of the resulting capacitor. Therefore, it is important to minimize the loss of specific surface area under sintering conditions.
In the production of tantalum capacitors, for example, tantalum powder is typically heated under vacuum to cause agglomeration of the powder while avoiding oxidation of the tantalum. Following this treatment, however, the tantalum powder often picks up a considerable amount of additional oxygen because the initial surface layer of oxide goes into solution in the metal during the heating and a new surface layer forms upon subsequent exposure to air, thereby adding to the total oxygen content of the powder. During the later processing of these powders into anodes for capacitors, the dissolved oxygen may recrystallize as a surface oxide and contribute to voltage breakdown or high current leakage of the capacitor by shorting through the dielectric layer of amorphous oxide.
As the technology of capacitors is continually demanding higher surface area valve metal powders, the requirement for oxygen management exceeds the effectiveness of the available methods of oxygen control. Accordingly, the electrical properties of capacitors could be improved if the oxygen content could be controlled, i.e., decreased or maintained about constant, during the powder processing.
One method to deoxidize valve metal powders, such as tantalum powder, is to mix alkaline earth metals, aluminum, yttrium, carbon, and tantalum carbide with the tantalum powder. However, the alkaline earth metals, aluminum, and yttrium form refractory oxides that must be removed, such as by acid leaching, before the material can be used to produce capacitors. Typically, the post-deoxidation acid leaching is performed using a strong mineral acid solution including, for example, hydrofluoric acid, at elevated temperatures of up to 100.degree. C. to dissolve the refractory oxide contaminants. The carbon content must be controlled because it may also be deleterious to capacitors even at levels as low as 50 ppm. Other methods have been proposed, including using a thiocyanate treatment, or a reducing atmosphere throughout the tantalum powder processing, to prevent oxidation and provide low oxygen content.
Other processes for controlling the oxygen content of valve metal materials, such as tantalum, niobium, and their alloys, include the use of getter materials. For example, Hard, in U.S. Pat. No. 4,722,756, describes heating the materials in an atmosphere containing hydrogen gas in the presence of a metal, such as zirconium or titanium, that is more oxygen active than tantalum or niobium. Another process for controlling the oxygen content of valve metal materials is disclosed by Fife, in U.S. Pat. No. 4,964,906. This process involves heating a tantalum material in a hydrogen-containing atmosphere in the presence of a tantalum getter metal having an oxygen concentration lower than the tantalum material. While these processes provide some control of the oxygen content of valve metal materials, there is a desire to improve the electrical properties of valve metal capacitors, particularly those formed from tantalum, niobium, and alloys thereof, by controlling, i.e., decreasing or maintaining about constant, the oxygen content of the valve metal powders. Accordingly, a demand exists for process improvements to reduce the oxygen content of these materials, particularly after they have been subjected to a deoxidation process.
In addition to the problems with powders and capacitor applications, high oxygen contents in fabricated wrought products of valve metals can decrease the ductility of the products.
It is therefore an object of the present invention to provide a method of controlling the oxygen content in valve metal materials. It is another object of the present invention to provide a method of controlling the oxygen content in valve metal powders, such as tantalum, niobium, and alloys thereof, useful in the production of capacitors, particularly after the powders have been subjected to a deoxidation process.